Ultraviolet type of flame detection equipment has been used for many years in various applications. One widely used application is in the detection of a flame in a burner with the detection equipment insuring proper and safe operation of the burner equipment. Ultraviolet flame detection equipment typically relies on a gas filled tube that has a potential impressed across the tube. In the presence of an ultraviolet source, the tube becomes conductive and allows current to be conducted. Circuits of this type typically use an ultraviolet responsive tube that is allowed to conduct, and is then quenched by the application of the alternating current potential. Circuits utilizing this equipment have typically been designed so that a short circuit of the tube causes an output signal which is not responded to as is a tube response to the presence of an ultraviolet source, such as a flame. Typically a short circuit causes equal conduction in both directions of an alternating current while the ultraviolet responsive tube normally conducts only during all or part of one half of the cycle of the applied alternating current potential. It was originally thought that this type of circuit was immune to a low resistance short circuit.
Inadvertently it was found that a relatively low resistance shunt could develop across an ultraviolet responsive tube if moisture was present across the tube leads. This type of failure occurs very rarely, but can create an unsafe condition in equipment that is designed for safe operation of burners and other types of devices that rely on an ultraviolet responsive tube for flame detection. The presence of a moisture condition across the leads of an ultraviolet responsive tube tended to provide a resistance value in the range of 7,000 to 60,000 ohms and is a rare type of failure. Even so, the possibility of this type of a failure is highly undesirable.